Patent Grant
8587788
This application claims priority to foreign French patent application No. FR 1001969, filed on May 7, 2010, the disclosure of which is incorporated by reference in its entirety.
The field of the invention is that of laser gyros used notably in the aeronautical field for inertial navigation, and more particularly to that of solid-state laser gyros.
Most laser gyros commercially available at the present time consist of a cavity made of zerodur containing a helium/neon gas mixture, in which cavity two counterpropagating optical waves coexist, i.e. waves propagating in opposite directions inside the cavity. It is known that in this type of laser, when the rotation velocities are low, the two counterpropagating waves have the same frequency. This frequency-locking problem at low rotation velocities or “dead zone” problem is usually solved by making the cavity vibrate about its rotation axis by mechanical dithering. A rotation velocity sufficient to prevent frequency locking is thus artificially created. This technique is however a non-inconsiderable source of noise because of the “random walk” phenomenon corresponding to the accumulation of a random phase at each pass through the dead zone.
To eliminate these drawbacks, another type of laser gyro has been developed. The physical principal consists in making not two but four waves coexist in the cavity, corresponding to two orthogonal polarization states and two opposite propagation directions. The patent by K. Andringa with reference U.S. Pat. No. 3,854,819 describes a device of this kind. This type of laser, also called a “multi-oscillator laser gyro”, associated with a magneto-optic frequency bias, makes it possible to eliminate the “dead zone” effect over the entire operating range of the laser gyro while still obviating, by an astute recombination of the four modes present in the laser cavity, fluctuations of said magneto-optic bias, which constitute without this a source with too large a drift for most laser gyro applications.
It is also possible to produce laser gyros with solid-state amplifying media, for example a laser-diode-pumped Nd:YAG crystal. The publication by S. Schwartz, G. Feugnet, P. Bouyer, E. Lariontsev, A. Aspect and J.-P. Pocholle published in Phys. Rev. Lett. 97, 093902 (2006) describes a laser gyro of this type.
In these devices, the competition between modes is no longer counterbalanced by the Doppler effect, as is the case in helium-neon gas laser gyros, but by an additional stabilizing device, for example the use of a feedback loop creating differential losses proportional to the difference in intensity between the counterpropagating modes of the laser, this being referred to as “active stabilization”. Such a device, although it proves to be relatively simple to implement for a solid-state laser gyro of the “two wave” type as described in the patent by S. Schwartz, G. Feugnet et J.-P. Pocholle with the reference U.S. Pat. No. 7,548,572, is however more complicated to implement in its “four wave” or multi-oscillator version as described in the patent by S. Schwartz, G. Feugnet and J.-P. Pocholle with the reference U.S. Pat. No. 7,230,686.
The laser gyro according to the invention is simpler to produce. It employs a multi-oscillator solid-state laser gyro no longer using an active stabilizing device acting on the differential losses, but a passive stabilizing device involving non-linear effects in the laser cavity, much simpler to implement.
More precisely, the subject of the invention is a laser gyro for measuring the angular velocity or the relative angular position along a defined rotation axis, said laser gyro comprising at least:
Preferably, the optical element is a birefringent crystal designed so as to generate, from a propagation mode having a first optical frequency, a light beam at a second optical frequency which is double the first frequency.
Advantageously, in a first configuration, the stabilizer device comprises two identical optical elements made of a birefringent crystal which are in the form of plates having parallel plane faces, the faces being perpendicular to the propagation axis of the various propagation modes, the two elements being assembled via a common face, and the crystallographic axis of the first element being perpendicular to the crystallographic axis of the second element. The optical element may be made of a barium beta-borate (BBO or βBaB2O4) crystal.
Advantageously, in a second configuration, the optical element comprises a stack of regularly alternating thin plane layers, of identical thickness, produced in the same negative uniaxial birefringent crystal, the faces of the various layers being perpendicular to the propagation axis of the various propagation modes, the optical axis of each layer being parallel to the plane of the layer and oriented in the same direction in the various layers, the polarization directions of the various propagation modes being oriented at 45° to said optical axis, and the sign of the effective non-linear coefficient of each layer being the reverse of that of the following thin layer. In this case, the thickness (Λ) of each layer is equal to Λ=2 mLC, m being an odd integer, LC representing the coherence length, LC being equal to
and n2 and n1 being the optical indices of the crystal at the second optical frequency and at the first optical frequency, respectively. The optical element may be made of lithium niobate (LiNbO3).
Advantageously, the amplifying medium acts as stabilizer device. In this case, the amplifying medium may be made of neodymium-doped yttrium calcium oxyborate (YCOB or YCa4O(BO3)3) or neodymium-doped yttrium aluminium borate (YAB or YAl3(BO3)4).
The invention will be greater understood and other advantages will become apparent on reading the following description given by way of non-limiting example and together with the appended figures in which:
The block diagram showing the principle of the invention is given in
Four propagation modes propagate inside the cavity, the first propagation mode and the third propagation mode being linearly polarized in the same direction, the second propagation mode and the fourth propagation mode being linearly polarized perpendicular to the first mode and to the third mode, the first propagation mode and the second propagation mode propagating in a first direction in the cavity, and the third propagation mode and the fourth propagation mode propagating in the opposite direction in the cavity.
The solid-state gain medium may for example be an Nd:YAG crystal pumped by a laser diode 15 as shown in
The magneto-optic bias generator provides the frequency splitting of the counterpropagating modes and thus eliminates the “dead zone” effect. As shown in
The eigenstates of the laser cavity thus defined are linear outside the magneto-optic bias generator and circular within it. The eigenvalues associated with these eigenstates are principally determined by three potential sources of influence;
The principle of the multi-oscillator laser gyro consists in adjusting the first two bias sources so as to frequency-split the four modes of the cavity in order to eliminate the dead zone and obtain a linear Sagnac response independently of the fluctuations of said biases. This principle is illustrated in
It should be noted that a non-planar configuration of the cavity in which the eigenstates of the cavity are circular may be used. Helium-neon multi-oscillator laser gyros may have configurations of this type. In such a case, the device for generating the magneto-optic bias is composed solely of the Faraday rotator, with no quarter-wave plates. The measurement principles remain the same.
As mentioned, the device for stabilizing the intensity of the four propagation modes having substantially equivalent levels comprises at least one optical element made of a non-linear crystal of the frequency-doubling type. The principle is the following: since the frequency doubling is proportional to the intensity of the initial mode, such a device will induce, for each of the modes in the cavity, losses that are higher the higher the intensity of the modes, and thus stabilize the bidirectional operation of the laser. To ensure that the four modes coexist in the cavity, the doubling is necessarily carried out on the two orthogonal polarization states corresponding to the eigenmodes of the laser. It is important for the doubled beams not to propagate inside the cavity, so as to prevent the principal modes from being parasitized. For this purpose, it is advantageous for the mirrors of the cavity to have a treatment intended to introduce large losses at the doubled frequency, that is to say the mirror treatment has a low reflection coefficient at the doubled frequency. This presents no technical difficulty in so far as, in principle, the doubled frequency is well away from the initial frequency.
Assuming that the doubled intensity per unit time may be expressed as KI/ISAT, K being the doubling coefficient, I being the intensity of the doubled mode and ISAT being the saturation intensity, then the self-stabilization condition may be expressed as K>γ/(2Ω2T12) in which γ is the level of losses per unit time, Ω is the frequency bias induced by the magneto-optic device and T1 is the life time of the excited level, which is of the order of a few hundred microseconds.
There are various possible configurations for producing the frequency-doubling device. As non-limiting examples, the configurations detailed below are produced from birefringent crystals. It is known in fact that, in order for frequency doubling to be possible in a given material, then the optical index of the material must be such that it satisfies a property called the phase tuning condition. The phase tuning condition forces the wavevectors {right arrow over (k)}ω and {right arrow over (k)}2ω associated with the fundamental angular frequency ω and the harmonic angular frequency 2ω to be equal, which is expressed as: {right arrow over (k)}2ω=2{right arrow over (k)}ω
where
being the refractive index at the angle of frequency of the field Ω. This results in the condition: n2ω=nω.
The laws governing the spectral dispersion of the refractive index of isotropic solid media do not allow the above equality to be achieved. Only by using anisotropic birefringent materials is it possible to obtain this condition. In a birefringent medium, the refractive index depends on the polarization direction of the light beam that passes through it. However, there is at least one preferential direction for which the index is independent of the polarization direction. Such a direction is called the optical axis of the medium. This is denoted as the axis Z in
Uniaxial media have two principal refractive indices, called the ordinary index and the extraordinary index. In general, they are denoted by no and ne, respectively. The index difference Δn, equal to ne−no, is then called the birefringence of the medium. This difference Δn is zero along the optical axis Z and is maximum for a direction perpendicular to this axis. Two cases may be distinguished according to the sign of the birefringence:
Δn>0: the medium is called a positive uniaxial medium;
Δn<0: the medium is called a negative uniaxial medium.
In what follows, it is more particularly devices employing uniaxial birefringent crystals that are addressed.
Knowing the variation of the various optical indices with the wavelength, it is possible to determine, for a given material and for propagation modes at known wavelengths, if ranges of orientation θ exist for obtaining phase tuning. Thus, by employing a barium beta-borate (BBO or βBaB2O4) crystal it is possible to obtain phase tuning enabling a frequency-doubling operation to be carried out. For this type of crystal, the internal phase tuning angle θAP is equal to about 23°.
The frequency-doubling stabilizer device must operate for both polarization states. To obtain this function, two crystals 21 and 22 mounted head to tail are inserted into the cavity, the crystallographic orientations of said crystals being turned through 90° so that the two polarization states of the counterpropagating waves circulating in the cavity are treated separately.
An alternative approach to phase tuning consists in using what are called QPT (quasi phase tuning) devices. As was mentioned, if the phase tuning is not perfect, frequency doubling cannot occur. However, it has been demonstrated that, if the phase mismatch is periodically zeroed, then frequency doubling can again occur. To achieve this property, the frequency-doubling device has a period structure 24 made up of crystalline layers 23 of the same material in which the sign of the effective non-linear coefficient of each layer is the reverse of that of the next thin layer, as illustrated in
The spatial period Λ of this reversal of the non-linear coefficient is: Λ=2 mLC where m is the order of the quasi phase tuning and equal to an odd integer and LC represents the coherence length such that:
where Λω is the wavelength of the non-doubled mode, nω is the optical index at the frequency ω, and n2ω is the optical index at the doubled frequency 2ω. The phase tuning condition associated with this interaction is: {right arrow over (k)}2ω−2{right arrow over (k)}ω−{right arrow over (k)}=0 where {right arrow over (k)} is the wavevector of the periodic lattice for reversing the non-linear coefficient.
As was mentioned, depending on the polarization state P of the field related to the principal plane, an ordinary input wave “o” and an extraordinary wave “e” may be defined. It is possible to obtain quasi phase tuning for these two waves. In the same way, the doubled output waves may be ordinary or extraordinary waves. Thus, depending on whether the input and output waves are all ordinary or all extraordinary waves, the interactions are referred to as “eee” or “ooo”. Thus, it has been demonstrated that, with a negative uniaxial LiNbO3 crystal, of order 1, the period is equal to Λ=6.807 μm for an “eee” interaction, which is the most effective. This corresponds to an optical axis perpendicular to the plane of the layers periodically alternated in terms of the sign of the non-linear coefficient, as shown in
where dij represents that element of the non-linear tensor in question which depends on the conditions for excitation along the o or e axes. Thus, by coupling the two counterpropagating waves to a non-linear crystal at 45° to the Z axis, the interaction can be made symmetrical, as illustrated in
Advantageously, the gain medium and the passive stabilizing crystal are made of one and the same doped material, which has the required non-linear properties for ensuring both amplification and frequency doubling.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 01969 | May 2010 | FR | national |
| Number | Name | Date | Kind |
|---|---|---|---|
| 3854819 | Andringa | Dec 1974 | A |
| 7230686 | Schwartz et al. | Jun 2007 | B1 |
| 7319513 | Schwartz et al. | Jan 2008 | B2 |
| 7446879 | Feugnet et al. | Nov 2008 | B2 |
| 7474406 | Feugnet et al. | Jan 2009 | B2 |
| 7548572 | Schwartz et al. | Jun 2009 | B2 |
| 7561275 | Feugnet et al. | Jul 2009 | B2 |
| 7589841 | Schwartz et al. | Sep 2009 | B2 |
| 7710575 | Schwartz et al. | May 2010 | B2 |
| 20040202222 | Pocholle et al. | Oct 2004 | A1 |
| 20090304033 | Ogilvy et al. | Dec 2009 | A1 |
| 20100123901 | Schwartz et al. | May 2010 | A1 |
| 20100265513 | Schwartz et al. | Oct 2010 | A1 |
| Number | Date | Country |
|---|---|---|
| 0 429 319 | May 1991 | EP |
| 0429319 | May 1991 | EP |
| 2 876 447 | Apr 2006 | FR |
| 2 925 153 | Jun 2009 | FR |
| 2007068652 | Jun 2007 | WO |
| Entry |
|---|
| Sylvain Schwartz et al., “Mode-Coupling Control in Resonant Devices: Application to Solid-State Ring Lasers”, Physical Review Letters 97, 093902-1-093902-4, Sep. 1, 2006. |
| Number | Date | Country | |
|---|---|---|---|
| 20110273720 A1 | Nov 2011 | US |
